a) Field of the Invention
The present invention relates to a large aperture photographic lens system for compact lens shutter cameras.
b) Description of the Prior Art
A lens system disclosed by Japanese Patent Kokai Publication No. Hei 3-265809 is a conventional example of a lens system. The system is a telephoto consisting of four lens components of four lens elements. The lens system has a triplet and a negative meniscus lens element which is disposed on the image side and has a convex surface on the image side.
Further, lens systems disclosed by Japanese Patent Kokai Publication No. Sho 59-177514 and Japanese Patent Kokai Publication No. Hei 9-211320 are known as conventional examples of the lens system. Such a system consists of the four lens components of five lens elements, and uses a cemented lens component as a third lens component of the triplet. The lens system has four lens elements of five lens elements, or the Tessar type lens system and a negative meniscus lens element.
The lens system disclosed by Japanese Patent Kokai Publication No. Sho 59-177514 and Japanese Patent Kokai Publication No. Hei 3-265809 have field angles on the order of 63.degree. at maximum and cannot be said as lens systems which have sufficiently wide field angles though these lens systems have sufficient high brightness or an aperture ratio of F/2.8.
Furthermore, the lens system disclosed by Japanese Patent Kokai Publication No. Hei 9-211320 has a simple composition consisting of the four lens components of five lens elements, a high aperture ratio of F/2.8 and a wide field angle on the order of 72.degree..
However, this conventional lens system is apt to produce distortion in the positive direction due to a fact that a stop is disposed between a third lens component and a fourth lens component, thereby making a power distribution remarkably asymmetrical with regard to the stop. In addition, this lens system allows curvature of field to be remarkably varied dependently on object distances and requires, for obtaining a favorable image located at a short distance, the so-called floating, or focusing by moving certain lens components for distances which are different from a moving distance of other lens components. Accordingly, this lens system must use a complicated lens moving mechanism. Further, the lens system can hardly obtain a sufficient amount of marginal rays due to a fact that the stop is disposed at a long distance from a first lens element, whereby offaxial rays transmit through the first lens element at heights far from an optical axis.
Furthermore, this conventional example is disadvantageous from the viewpoint of lens surface processing since it uses at least two aspherical surfaces on the fourth lens element and another lens element before the fourth lens element.